Platinum-Doped Anatase (101) Surface as Promising Gas-Sensor Materials for HF, CS2, and COF2: A Density Functional Theory Study.

In order to find promising sensor materials for HF, CS2, and COF2 detection to realize the online internal insulation defect diagnosis of a SF6 gas electrical device, the gas sensing property, binding energy, adsorption distance, charge transfer, and density of states distribution, of Pt-doped anatase TiO2 (101) surfaces on HF, CS2, and COF2 gas molecules was calculated and analyzed in this paper based on the density functional theory. The work suggested that the Pt-TiO2 surface has a nice gas sensing upon CS2 and COF2 because of the increase of the conductivity of the Pt-TiO2 surface and the suitable adsorption parameter after CS2 and COF2 adsorbing on it. However, this material is not suitable as a gas sensor for HF gas. All of the works provide theoretical adsorption information of Pt-TiO2 as a gas sensor material for HF, CS2, and COF2 detection.

Introduction

SF6 gas has been widely used as insulation media in the electrical industry with excellent insulation and arc extinction properties.[1−4] However, it could decompose and react with trace moisture, O2, and epoxy resin material in the SF6 gas electrical device to generate CO2, CF4, SOF2, SO2F2, CS2, HF, COF2, etc. under partial discharge.[5−7] To guarantee the safe operation of the gas electrical device, the analysis of chemical gas sensors is performed for on-line detection of SF6 characteristic decomposition gases to diagnose the insulation-mode SF6 gas electrical device by SF6 decomposition product analysis.[8−11]

Considering the remarkably high catalytic property,[12,13] the exploration of noble metal-doped anatase titania (101) for the detection of some SF6 decomposition components of online internal insulation defect diagnosis of a gas electrical device via decomposition products analysis has been studied,[14−16] and Pt doped TiO2 has a nice gas-sensitivity performance on SOF2, SO2F2, SO2, NO, CO, and N2O4.[17−21] However, there are a few of studies about the gas-sensing property of some key SF6 characteristic decomposition components on noble metal-doped TiO2 nanotube materials, that is, HF, CS2, and COF2, which could better indicate the internal insulation defects of the gas electrical device.[22−25] The contents of CS2 could be 12 μL/L and those of COF2 could be 80 μL/L during the operation of the gas electrical device. In this paper, in order to study the gas-sensing characteristics of Pt-doped anatase TiO2 (Pt-TiO2) for HF, CS2, and COF2, we calculated and analyzed the adsorption property, including binding energy, charge transfer, and density of states.

Computational Methods

All the simulations were implemented by the Dmol3 package based on the density functional theory (DFT),[26−28] Perdew–Burke–Ernzerhoff (PBE) function with general gradient approximate (GGA) was used to deal with the electron exchange and correlations.[29] Double numerical plus polarization (DNP) was utilized as the atomic orbital basis set. The Tkatchenko and Scheffler (TS) DFT-D was used to deal with the dispersion forces.[30] For the convergence criteria, the energy tolerance accuracy was selected at 1.0 × 10–5 Ha. The maximum force was chosen at 0.002 Ha/Å. Also, the maximum atom displacement was chosen at 5 × 10–3 Å. The smearing is 0.005 Ha.[31] The self-consistent field (SCF) tolerance of 1.0 × 10–6 Ha and global orbital cut-off radius of 5.0 Å were utilized to guarantee the accurate calculation of total energy.[32] In addition, the k-point sample of the Monkhorst–Pack grid was sampled to 2 × 2 × 1 at the Brillouin zone for geometric optimization.[19] The binding energy E for the adsorption system after gas molecules adsorbing on Pt-TiO2 is shown in eq (33,34)

In the formula, Esur is the energy of the isolated Pt–TiO2 surface. Egas is the energy of gas molecule before adsorption. Also, Egas+sur is the energy of the adsorption system after gas molecules adsorbed on the Pt–TiO2 surface.

Results and Discussion

The doping position of the Pt atom is based on the previous study,[19] namely, one platinum atom form two Pt-O bonds with the oxygen atom of the outer surface of Pt–TiO2. The geometric structures of Pt–TiO2 are shown in Figure . The supersize of the Pt–TiO2 surface is 10.88 × 11.33 × 19.35 Å with a 12 Å vacuum layer. The concentration of the Pt dopant is 1/120. In addition, comparing with the Ti5c site and O2c site, the Pt site of Pt–TiO2 are more favorable for the adsorption of gas molecules.

Figure 1

Views of the Pt-doped TiO2 (101) surface.

Adsorption Property of HF on Pt–TiO2

For HF gas molecules, there are two adsorption modes; that is, the H atom and F atom approached the Pt site of the Pt–TiO2 surface, as shown in Figure . For the F atom adsorption mode, the binding energy and adsorption distance are −0.241 eV and 2.754 Å, respectively, with the electrons of 0.006 e transferring from the HF molecule to the Pt–TiO2 surface. However, the Pt–TiO2 donates 0.004 e electrons to HF in the H adsorption mode with a binding energy of −0.170 eV and an adsorption distance of 2.256 Å.

Figure 2

Optimized adsorption structure of the: (a) F atom adsorption mode of HF on the Pt–TiO2 (101) surface; (b) H atom adsorption mode of HF on the Pt–TiO2 (101) surface.

In addition, the total density of state (TDOS) and partial density of state (PDOS) distributions of HF absorbed on Pt–TiO2 are shown in Figure . One can observe that both TDOSs of F and H atom adsorption modes resemble the TDOS of isolated Pt–TiO2, except that there is a novel small peak appearing around −9 eV for the F atom adsorption mode and a novel small peak appearing around −7.5 eV for the H atom adsorption mode. The pseudogap and TDOS values under Fermi level slightly changed before and after adsorption. As for the PDOS, the hybridization between the 2p orbital of F and 5d orbital of Pt is weak. Moreover, it could be found that the novel peak of TDOS after adsorption is mainly contributed by the 2p orbital of the F molecule.

Figure 3

TDOS and PDOS for the HF adsorption system: (a) F atom adsorption mode and (b) H atom adsorption mode.

Adsorption Property of CS2 on Pt–TiO2

Figure shows that the CS2 adsorbs on the Pt–TiO2 surface, where two adsorption modes are considered. As for the C adsorption mode, CS2 donated 0.005 e electrons to the Pt–TiO2 surface in the adsorption process with a binding energy of −1.594 eV and an adsorption distance of 2.120 Å. At the same time, the bond angle S–C–S of the CS2 molecule changes from 180 to 150.508°. As for the S adsorption mode, the calculation adsorption distance is 2.463 Å and the binding energy is −1.596 eV. There are 0.003 e electrons transferred from the CS2 gas molecule to the Pt–TiO2 surface by the Mulliken population analysis.

Figure 4

Optimized adsorption structure of the: (a) C atom adsorption mode of CS2 on the Pt–TiO2 (101) surface and (b) S atom adsorption mode of CS2 on the Pt–TiO2 (101) surface.

The DOS configurations of CS2 adsorbed on Pt-TiO2 (101) surface are shown in Figure . Both the TDOS of C and S adsorption modes shifted 1 ∼ 2 eV to the right as a whole markedly after adsorption. In addition, there are three novel peaks appearing in the TDOS configurations of both adsorption modes around −15, −13, and – 7 eV. The pseudogaps of 1.068 eV in S adsorption mode and 1.063 eV in the C adsorption mode are quite larger than those of the isolated Pt–TiO2 surface with 0.557 eV. It could be hypothesized that the covalence of the material would increase after CS2 gas molecules adsorb on the material surface, and the material probably has a fairly nice gas sensitivity to CS2.

Figure 5

TDOS and PDOS for the CS2 adsorption system: (a) C atom adsorption mode and (b) S atom adsorption mode.

Adsorption Property of COF2 on Pt–TiO2

Figure shows the optimized adsorption structure of COF2 on the Pt–TiO2 surface. In the O adsorption mode, the binding energy is −0.308 eV, the adsorption distance is −2.611 Å, and the COF2 gas molecule transferred 0.002 e electrons to Pt–TiO2. As for the F adsorption mode, the binding energy and adsorption distance are −0.866 eV and −5.360 Å, respectively, with the COF2 gas molecule donating electrons of 0.002 e. In addition, for the C adsorption mode, the binding energy and adsorption distance are −0.506 eV and −2.164 Å, respectively. The charge transfer is the same as that in the F atom oriented system.

Figure 6

Optimized adsorption structure of the: (a) O atom adsorption mode of COF2 on the Pt–TiO2 (101) surface; (b) F atom adsorption mode of COF2 on the Pt–TiO2 (101) surface; and (c) C atom adsorption mode of COF2 on the Pt–TiO2 (101) surface.

The DOS distributions of the COF2 molecule absorbed on the Pt-doped TiO2 (101) surface under three kinds of adsorption modes is shown in Figure . One can observe that the TDOS distributions of O and F adsorption modes resemble the TDOS of the isolated Pt–TiO2 surface, except that three novel peaks appeared near −12.5, −11, and −9 eV. Comparing with PDOS distributions, the 2p orbital of the F atom is the main contributor of three novel peaks. However, the TDOS distribution of C adsorption mode is shifted to right in contrast to that of isolated Pt–TiO2. In addition, the overlapping areas between the 2p orbital of the C atom and the 5d orbital of Pt in the C adsorption mode is significantly larger than those in the O and F adsorption modes. Both pseudogaps of the O adsorption mode with 0.546 eV and F adsorption mode with 0.635 eV are a little different from that of the isolated Pt–TiO2 surface with 0.557 eV. However, the pseudogap of the C adsorption mode with 1.021 eV is quite larger than that of the isolated Pt–TiO2 surface. Consequently, COF2 gas molecules are most easily adsorbed by the Pt–TiO2 surface by the C adsorption mode in three kinds of adsorption modes. Moreover, it could be hypothesized that the covalency of Pt–TiO2 may increase with the adsorption of the COF2 gas molecule.

Figure 7

TDOS and PDOS for the COF2 adsorption system: (a) O atom adsorption mode; (b) F atom adsorption mode; and (c) C atom adsorption mode.

In a word, due to the increased conductivity, the Pt-doped TiO2 surface could be used as the gas sensor material to detect COF2 and CS2. However, considering the results of calculation, this material should not be applicable to detect HF gas accurately.

Conclusions

In this paper, several parameters of HF, CS2, and COF2 adsorbing on a Pt-doped anatase TiO2 (101) surface were simulated to evaluate the feasibility of this material for the detection of HF, CS2, and COF2 based on DFT. Considering the adsorption mode and DOS distributions, it could be concluded that the Pt-TiO2 surface has a nice gas-sensitivity performance on CS2 and COF2. However, this material is not suitable to be used as a gas sensor for HF.